The present disclosure relates to an image reading device.
A generic image reading device detects a size of an original document placed on a document table thereof before reading an image of the original document. The image reading device includes a light source. The light source irradiates the document table with light before a cover of the image reading device is closed toward the document table. In a position of the original document on the document table, the light emitted by the light source is reflected off the original document and becomes incident on an imaging element. In a position without the original document, on the other hand, the light emitted by the light source passes through and out of the document table. The position of the original document is therefore detected by detecting the light received by the imaging element. A size of the original document is detected based on the position of the original document.
However, ambient such as light from a fluorescent lamp may fall on the document table before the cover is closed. That is, the imaging element may receive such ambient light. In such a situation, the position of the original document is detected based on the ambient light received by the imaging element. As a result, the image reading device may erroneously detect the size of the original document.
In order to deal with the problem, an image reading device varies the emission intensity of light that is emitted by a light source therein so that the resulting temporal variation is different from the temporal variation of the emission intensity of ambient light. When the temporal variation of the amount of light received by the imaging element is consistent with the temporal variation of the amount of light emitted by the light source, therefore, it is determined that an original document is present on the document table, and the position of the original document is detected. The size of the original document is then detected based on the position of the original document.